The Vivelle instructions say not to put it somewhere where it might be rubbed/chafed by clothing, so maybe a few of them are getting scraped off by your underpants waistband or something? Vivelle's adhesive formulation *supposedly* hasn't changed... although we never know, with pharmaceutical companies.
You can also try very carefully cleaning the area with soap and water and then a second time with rubbing alcohol before applying the patch, in case it just isn't grabbing the skin very well.